This invention relates to a technique for forming fine-line patterns on a substrate and, more specifically, to a method for improving resist adhesion during a processing sequence in which large-scale-integrated (LSI) circuits are fabricated on a semiconductor wafer.
As is well known, resist layers capable of being patterned in a high-resolution way are widely utilized to fabricate LSI circuits on a semiconductor wafer. Initially, patterns are defined in a resist layer adhered to an underlying substrate by irradiating selected portions of the layer with light, electrons, ions or X-rays. The irradiated portions are then removed (positive resist) or not removed (negative resist) from the underlying substrate in a developing step. Subsequently, the uncovered portions of the substrate are processed, for example etched, in a standard pattern transfer step.
In practice, the adherence between edge portions of the resist and the substrate is often weakened or destroyed during the developing step or during the aforenoted subsequent processing step. In turn, this frequently leads to lifting or actual removal of these portions. The edge acuity of the resist pattern is thereby deleteriously affected. In many cases of practical interest, particularly as feature sizes in LSI circuits become increasingly smaller, this loss in edge acuity leads to a significant reduction in the yield of acceptable chips from a wafer.
The problem of achieving good adhesion between a resist layer and an underlying substrate is a general one. But the problem of reliably adhering phenol-formaldehyde positive photoresists to underlying surfaces made of, for example, phosphorus-doped silicon dioxide or silicon nitride is a particularly vexing one. For such surfaces, the use of a very thin (1-to-2 Angstrom) layer of a surface modifier such as hexa-methyl disilizane (HMDS) as an adhesion promoter for positive photoresists has been tried. But, in practice, it has been observed that HMDS often only marginally improves the adhesion of such photoresists to such surfaces.
Accordingly, continuing efforts have been made by workers in the LSI circuit art directed at trying to improve the adhesion achieved between resists and underlying surfaces. It was recognized that such efforts, if successful, could significantly improve the yield and thereby lower the cost of LSI circuits.